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New Paradigms in Magnetic Recording: Understanding through Micromagnetic Modeling. i l Martin Plumer. Department of Physics and Physical Oceanography, Memorial University. (Design engineer, Seagate Technology 1997 - 2005) 1. Overview of magnetic recording: - Head (Write Element & Read Element-GMR spin valve) and Recording Medium (disc). - Fundamental Limitations: Superparamagnetism (a Bad thing). 2. Micromagnetics: 2 dM (1 ) M H eff M M H eff LLG equation dt M P bl THE Problem: Long-Range Magnetic Dipole Interactions i i l i 3. Micromagnetic Recording Model: Writer/Reader/Media - Impact of Recording Medium effects (disc microstructure) on the quality of recorded bits. What s 4. What’s New: Perpendicular and Tunneling. 5. And What’s Next ? Reduction in growth rate due to superparamagnetism 2006 Self Organized Magnetic Arrays Heat Assisted Magnetic Recording 40% 100% AD=(1/track width)*(1/bit length) =(tracks per inch)(bits per inch) • Today: AD ~ 300Gb/in2 Bit Eric Meloche length ~ 3 media grains ~ 200 Å February 14, 2008 • Tomorrow: AD ~ 1000Gb/in2. Bit length ~ 1 media grain ~ 50 Å Press Release. Source: Western Digital Technologies WD(R) Launches Industry s First 2 TB Hard Drives Tuesday January 27, 2009. WD's Eco-friendly, Cool and Quiet, WD Caviar® Green(TM) Drive Marks the Largest Capacity Hard Drive in the Industry LAKE FOREST, Calif. Jan. 27 /PRNewswire-FirstCall/ -- WD (NYSE: WDC - News) today world's announced the first 2 terabyte (TB) hard drive - the world s highest capacity drive and the latest addition to WD's popular, environmentally friendly, cool and quiet, WD® Caviar® Green(TM) hard drive family. This new 3.5-inch platform is based on WD's industry-leading 500 GB/platter technology (with 400 Gb/in2 areal density) with 32 MB cache, producing drives with capacities of up to 2 TB. The three elements of magnetic recording. 1. Overview a GMR Read Inductive Giant Magneto-Resistive Write Element Sensor Longitudinal Recording. Media d D= grains NiFe W GAP 8-10 nm N S S N N S S N N SS N N S B Recording Medium One ‘bit’ of information AD=(bpi)(tpi) AD (b i)(t i) bpi ~ 1/B tpi ~ 1/W Recording Head 1. Overview X-section and ABS view of an Integrated MR Head media SEM image Top pole Coils X write gap WRITER Magneto Return pole Top pole Resistive gap (MR) shared pole Bottom shield shields reader element View from the air bearing surface (ABS). ABS: Air Bearing Surface Read Sensor ~ 10 m Finite Element Method: Magsoft 2. Ferromagnets 2. Micromagnetics Not Micromagnetics g Coils generate ~ 200 Oe – Solves Maxwell’s Equations. 1.0 10,000 NiFe 1 0 T ~ 10 000 Oe Left side not shown. The business end 2.4T CoFe ABS Field in Media Hm ~ 10,000 Oe. Gap ~ 20,000 Oe MR Head: Writing A Fresh Track- 1. Overview Reversing the media magnetization What’s good for a write field profile. 1. Large field at center Hm top return pole pole 2. Large dH/dx Writer gap g p ~ 100 nm Down-track direction x M M ABS HMS (Head-To-Media Separation) ~ 15 nm Media ~ 10 nm magnetic layer H Recording a transition 8/19/97 HSE GMR (Giant Magneto-Resistive) Read Sensor 1. Overview 2007 Nobel Prize in Physics: Fert and Grünberg 1 GMR transfer curve 0.75 ~ 300 Å 0.5 2) R (Rfull/2 0 25 0.25 0 0 45 90 135 180 -0.25 -0.5 media -0.75 V ~ 4mV Current flows In the -1 Plane (CIP), mainly through Cu layer. AFM = NiMn (High TN). 200 Å Thin Films TEM PL = CoFe (big moment). 25 Å Fe Permalloy 25 Å FL = Ni80F 20 P ll GMR effect involves surface and bulk scattering of spin polarized electrons ABS view between the Free Layer and Pinned Layer. 1. Overview The Medium: A “hard” granular magnet Lubricant Carbon C b Need ΔE >> kBT Co-alloy CrV CoCrPt NiAl glass substrate Grain size ~ 16 nm Grain size ~ 9nm Energy barrier ~ ( i volume)*(crystal anisotropy) E b i (grain l )*( t l i t ) Media M-H loop 0.0003 0.0002 1 Gbit/in2 media 50 Gbit/in2 media 0.0001 M 0 -10000 -5000 0 5000 10000 -0.0001 -0.0002 -0.0003 Hc H SUPERPARAMAGNETISM: Smaller grains are worse for thermal stability SNR ~ N: Smaller grains are better. Hc ~ (M)(anisotropy)(grain volume) Noise sources = head electronic + media transitions 1. Overview Media SNR is the biggest limiting factor to smaller bit size. (Number of grains per bit): Smaller grains are better. SNR ~ Recording 330kfci 1.2G/s process breaks g g Need: grain size << bit length down above 1.4G/s 1 4G/s 390kfci 1.4~1.5G/s due to combination 445kfci 1.6G/s of high freq MFM T k I Track Image vs. Li Linear D it Density writing and 500kfci 1.8G/s high fly height. and Data Rate (at the fixed RPM 20krpm) 525kfci 1.9G/s D 2.0G/s AABdis d i illustrating SNR degradation Data ill i 550kfci operating with increasing linear density at 2.2G/s 180% of it’s 608kfci designed 635kf i 635kfci 2 3G/ surface 2.3G/s velocity. 663kfci 2.4G/s kfci=kilo-flux changes per inch (1000 per inch) G/s = Gigabits/second 1. Overview The Trilemma of shrinking dimensions. 1. Smaller bits require smaller media grains to maintain SNR. 2. Smaller grains require larger anisotropy (energy barrier) to maintain thermal stability. 3. Larger anisotropy requires larger write fields to switch media transitions. transitions • Also: Smaller dimensions lead to less responsive read elements (magentically). • Larger write fields require large moment materials to put at the business end of the it l th write element.t • CoFe at 2.4T is the largest moment material stable at room temperature and has been used since 1999. Micromagnetics to the rescue (and perpendicular recording and tunneling magnetoresistive read elements, …). 2. Micromagnetics g g Micromagnetics of Ferromagnets: Interacting, uniformly g, y magnetized grains. Thin ferromagnetic films with small shapes and big demagnetization (magnetostatic) fields at edges. s Cartoon of a thin film C t f thi fil G i size ~ 10 nm ﬂ 1000’ of atoms. Grain i 1000’s f t 3. MRM 2. Micromagnetics Landau-Lifschitz-Gilbert Equation Precession and phenomenological damping: Torque Equation Add damping dM dM ( r , t ) M H H (r , t ) H eff (r , t ) dt dt 2 dM LLG: (1 ) M H eff M M H eff = M dt M ~ 0.1 – 0.01 Norm conserving: Spin-wave interactions, magnetoelastic p , g 2 coupling, impurities,… dM dM 2M 0 dt dt a (a b ) 0 2. Micromagnetics LLG: Solutions for single moment M of NiFe in external field H=1000 Oe. 2 dM (1 ) M H eff M M H eff dt M =0 =0.1 =1 M MH M MH M H H H Precession only P i l Slo relaxation Slow rela ation relaxation Fast relaxation. H For H||z and M||x: Use large for f 2.8GHz -MxH ~ y MxH steady-state steady state 2 -Mx(MxH) ~ z solution. Magnetic Modeling Magnetic Films. Jason Mercer and John Whitehead (MUN). dM (1 2 ) M H eff M M H eff dt M y g 16x16x16 system of magnetic dipoles with periodic BCs on the side and open BC’s top and bottom, solved with the damping. torque equation plus damping Video shows the system evolving towards equilibrium after being given a random initial configuration. Colors represent different moments. orientations of the moments John Whitehead. 3. MRM 2. Micromagnetics Interacting Grains: The “Field” in LLG Each grain M(r) is acted upon b an effecti e field with pon by effective ith numerous contributions: Heff (r)= -E/M(r) i b Interacting bar magnets H effective H applied H anisotropy H exchange H magnetosta tic E.g., Writer Very strong in Long-Range NN only demagnetization head field Media Very Media. NN H ex (ri ) JS / M s M j 2 2 fields increase weak in Head j computational materials. Ferromagnets: Mi || Mj demands. Heff = Heff[M(r)] is a functional of M(r): Self-consistent solution required. 2 dM (1 ) M H eff M M H eff dt M 2. Ferromagnets 2. Micromagnetics g g g Ferromagnets: Cannot ignore “magnetostatic” fields. M (r ' )( r r ' ) n(r ' ) M (r ' )( r r ' ) ˆ H ( r ) d 3 dA 3 V | r r '| S | r r '| For grains of finite size, Long range interaction ~ 1/rn it’s not just dipole-dipole Increases computational demands. M2 Adjacent bar magnet lies anti-parallel • Outside of bar M=0; H = B = stray field S N • Inside of bar B = H + 4 Mr, ‘demagnetizing’ field H opposes M 2. Ferromagnets 2. Micromagnetics Shape Anisotropy: Simple Argument • Dipole-dipole interaction (first terms in multipole expansion) mi m j 3(mi rij )(m j rij ) E=-MH E 3 pairs r ij r 5 ij ij • Consider energy of dipole 1 (lattice spacing a=rij) a3E 2 (cos 12 cos 13 cos 14 cos 15 ) 3 m 2 1 4 3(cos 1 cos 2 cos 1 cos 4 sin 1 sin 3 sin 1 sin 5 ) ˆ y Put i 5 4 3(2 cos2 2sin2 ) 2 Completely isotropic. ˆ x 2. Ferromagnets 2. Micromagnetics Shape Anisotropy: simple argument g ( ) • Create edge (remove 3) a3E 2 1 4 2 (cos 12 cos 14 cos 15 ) m 5 ˆ y 3(cos 1 cos 2 cos 1 cos 4 sin 1 sin 5 ) Put i 3 3(2 cos2 sin2 ) 3 3cos2 3sin2 ˆ x Uni-axial i t induced b edge. U i i l anisotropy i d d by d Energetically favorable for spins to align parallel to the edge and to neighboring spins. 2. Ferromagnets 2. Micromagnetics Patterned Devices: Shape Anisotropy • Series of solutions for platelets with different aspect ratios: what do we see? – CoFe, 2.5 nm thick, 10 nm cells (single layer) Micromagnetic Modeling M t t d t li ll l t d •Moments tend to lie parallel to an edge. •Corners are complicated. Steady State Solution 2. Ferromagnets 2. Micromagnetics GMR spin valve response to a media transition field. H Field from media transition decays quickly Quiescent state (H=0) y Response to field from a media ABS transition FL FL ABS ABS Shape anisotropy limits response for smaller devices. Sh i t li it f ll d i DV=1mV, PL not 4mV. PL Thermal Fluctuations. 4. Spin 2. Micromagnetics G d o eC oc so o ccou o e uc u o s. LLG and Monte Carlo can also be used to account for thermal fluctuations. temperature Thermal averaged magnetization vs temperature. 2D square lattice with 2D square lattice. J=K=1 (Ising model). 1.2 strong axial anisotropy. · · · · · · · · · 1 LLG M MC · · · · · · · · · 0.8 · · · · · · · · · 0.6 06 •Temperature fluctuations 0.4 reduce the thermal average magnetic moment <M>. 0.2 •This leads to thermal noise, 0 0 0.2 0.4 0.6 0.8 1 reduction of AF/F pinning, … p g, T/J Transition Temperature More fun with Micromagnetics 2. Micromagnetics Spin waves in thin films Media M-H Hysteresis Loops - finite temperature - finite temperature - damping - sweep rates (VSM very slow, Disc Drive very fast) Magnetostatic mode Theory vs LLG for 3 ferromagnetic layers. ky=0. g/J 0.2, 0.15 g/J=0 2 Hy=0 15 3. MRM 3. Micromagnetic Recording Model 2) Write on LLG 3) Generate field from 1) Finite Element Method transition. (FEM-commercial micromagnetic Perform read back with software) write head field model of medium. f di micromagnetic reader model. •Use to write single-tone 0.8 patterns. patterns 0.6 06 0.4 0.2 Voltage (mV) • Pseudo-random sequence 0 0.45 0.55 0.65 0.75 0.85 0.95 -0.2 -0.4 require very long patterns -0.6 -0.8 (computationally intensive). down track (um) 2D contours of a write field at the media plane Longitudinal Recording: M in the plane of the disc. 3. MRM Media Transitions: Small noise on top of Big noise. bits i h 343000 bit per inch = 74 nm 2D in-plane anisotropy 64x64 grains Where s Where’s the transitions ? 3. MRM Longitudinal Media Transitions. 343000 bits per inch g g Alternating average magnetization between transitions Down track component only: bright dark signa voltage al 3. MRM Medium Orientation Effects • Orientation Ratio (OR) accounted for by assuming a Gaussian distribution for the media grain anisotropy axes di ti HK: mean and St d d Deviation. direction d Standard D i ti z x Isotropic media – very large standard deviation Oriented media – ‘smaller’ standard deviation (SD). p performance for media Expect better p with a preference for M to lie in the down-track direction. 3. MRM 100Gb/in2. Recorded tracks at high kbpi much improved with oriented media. Isotropic media. Oriented media. OR ~ 1/SD 290 and 640 kbpi. . 640 kbpi. Amplitude vs OR 290 kbpi OR=1.23 OR=1.6 640 kbpi g y g p A clear message was received by recording media development groups. Highly oriented media is now the industry standard. 3. MRM Micromagnetic Calculation of Signal-to-transition-noise ratio Single-tone pattern. L V ( x) dx 2 Bertram formula: SNR = 10 log L 0 = Amplitude V 2 ( x) V ( x) 2 dx i Noise 0 Average over 50 random configurations of the medium A d fi ti f th di anisotropy direction and magnitude. g Magnetization Profile 430 kfci, 100 ktpi 30 c , 00 tp g Read-back Voltage Better SNR is the key driver for higher areal density 3. MRM Effects of Medium Anisotropy Hk distribution 640 kbpi Hkstd=0 V Hk dV/dx SNR vs Hk std dev. F ll Micromagnetic calculation of Transition Fully Mi ti l l ti fT iti Hkstd=10% Hk d 10% SNR vs Hk distribution. TPWG=0.2 um. Hk = 9000 Oe. 430 kbpi. 100 ktpi. 17 16 15 14 SNR (dB) 13 12 Hkstd=20% 11 10 9 8 0 500 1000 1500 2000 Hk_std (Oe) A clear message: Distributions in media properties are bad for performance. 3. MRM Medium Grain Size Impact on Single-tone SNR vs kfci. MRM for Yellowstone 180 ktpi. Effect of media grain size on modeled single-tone media SNR. 17 Huge benefit from using 16 smaller mean grain size 15 on SNR, especially at 14 density. higher linear density SNR (dB) 13 12 11 Data illustrating SNR 10 10 nm degradation with 9 increasing linear density 8 nm 8 6 nm 7 300 400 500 600 700 kfci Pseudo-random sequence 4. What’s New Comparison of Longitudinal & Perpendicular Recording Longitudinal Recording Perpendicular Recording BL BL L P To help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content. To help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content. g Magnetostatic fields Magnetostatic fields destabilize the transitions stabilize the transitions. (superparamagnetism). 4. What’s New Larger Write Fields due to media soft underlayer. • 1 Longitudinal Recording • 1 Perpendicular Recording Deep Gap Field HG Auxiliary Main Pole Pole L P t Field in Medium HM K Media Soft Under Layer (Fringing Field) Double Keeper, 100 Layer Medium Deep Gap Field HG Single layer medium (longitudinal or perpendicular): Double layer perpendicular media with soft magnetic • Transition is recorded by fringing field Underlayer: media becomes part of the write element • Transition is recorded by deep gap field 4. What’s New …and more field emanating from media transitions GMR read sensor Stronger fields from perpendicular St fi ld f di l bits = larger play-back amplitude. boost Another ~50% boost. And…Tunneling GMR Heads: 4. What’s New Double the read-sensor amplitude. ( •New Structure aka: TMR, MTJ, SDJ,Tunnel valve, spin tunneling head) •New materials TOP SHIELD / ELECTRODE N h i •New physics Energy scale STABILILITY LAYER GAP ELECTRODES FREE LAYER TUNNEL BARRIER SAF PINNED LAYER BOTTOM SHIELD / ELECTRODE ield Shi Spin direction Tunnel barrier eld is conserved Incident electron Shie barrier during tunneling wave function height CPP TEM: ABS view Transmitted Ferromagnet Al2O3 Ferromagnet electron 1 barrier 2 wave function CPP = Current Perpendicular to Plane Distance 5. What’s Coming Then What Th Wh t ? 5. What’s Coming ECC - Exchange Coupled Composite Media Happl Soft: low Hk • Magnetic hard layer (high Hk) provides the thermal stability Hard: high Hk • Magnetic soft layer (low Hk) functions as a ‘lever’ to help switch the hard layer during writing. Hexc g g Switching Field vs Field Angle Victora and Shen (2004) Eric Meloche February 14, 2008 5. What’s Coming HAMR – Heat Assisted Magnetic Recording Drive Temperature civity C oerc Heat Store Media Here Cool Media Available Head Field Write Here Temperature • The challenge in making HAMR work is in creating sufficient thermal gradients that will prevent interference between adjacent bits. Eric Meloche February 14, 2008 5. What’s Coming Patterned Media Lithographically Patterned Bits, single magnetic grain to j decrease transition jitter and increase SNR. diameter C /C bil 100 nm di dots ith i d t Co/Cu bilayer d t with 200 nm period 5. What’s Coming Or… Self-Assembled Arrays. Self Assembled Magnetic Nanoparticle Arrays An illustration of nanoparticle self-assembly via solvent evaporation. Solvent Evaporation Fig.9.11. TEM images of (a) a 2D assembly, of the 8 nm cobalt nanoparticles on an amorphous carbon surface Co or FePt. Collaborators at Seagate: Collaborators at MUN: Johannes van Ek (now at WDC), John Whitehead, Jason Mercer, Heinonen Eric Meloche and Olle Heinonen. Trinh Nguyen Physical Oceanography Soft Matter Biophysics Polymers Photonics Magnetism Sensors and Actuators Computational Science http://www.mun.ca/physics/ Environmental Science Magnetic Materials • Carbon 3-4nm • Magnetic layer ~20nm (CoCrPt) Interlayers •I l • SUL 80-200nm Eric Meloche February 14, 2008 Writer Risetimes • Goals: Determine the most important factors that limit flux rise times throughout the entire pole structure Simple Pi t Si l Picture • Perpendicular component of magnetization in the interior of the writer has a fast response to coil field AB C Points B and C - Fast response time • Magnetization response slows down near the ABS Point A - Slow response time Time resolved optical measurement of Pole i ti ti P l region magnetization Coil current Eric Meloche February 14, 2008 M.R. Freeman et al., J. Appl. Phys. 81 (1997) Micromagnetics: Interacting grains. 2. Micromagnetics Exchange in terms of spins: NN E ex J ij S i S j JS / M s M Mj 2 2 i j ij ij j atoms J’’ grains MA=(MS/S)(S1+S2) ( )( A 1 2 3 4 B (M MB=(MS/S)(S3+S4) J’ Eex = -(J’S1 S2 + J’S3 S4 + J’’S1 S3 + J’’S1 S4 + J’’S2 S3 + J’’S2 S4) Exchange in terms of grains: a constant Eex = Eex(intra-grain) - J’’(S2/Ms2)MA MB J’ This approximation works provided J is strong enough so that S1 || S2. Magnetic grain size is determined roughly by the “exchange length” ~ 5 – 15 nm. p p g Model for Spin Dependent Scattering Density of States for Transition Metal Electrons Energy Empty M States 4s 4s 3d Fermi Filled States 3d minority carriers (spin antiparallel to majority carriers (spin parallel to M) M) can scatter into more unoccupied can scatter into fewer unoccupied states giving them higher resistivity. states giving them lower resistivity. LLG media M-H loop. 3. MRM Anisotropy axes of grains are randomly oriented in plane: • A i f i d l i di l 2D isotropic media. Hk=6400 Oe. Hc ~ Hk/2 1495_2: fac=0.055, Ms=350, Hk=6400, exh=0 1 0.8 M || H 0.6 0.4 0.2 H M/Ms 0 H M -1 0 -0.8 -0.6 -0.4 -0.2 2 0 -0.2 0.2 0.4 0.6 0.8 1 -0.4 Media M-H loop -0.6 0.0003 -0.8 0.0002 1 -1 0.0001 M || H H/Hk M -10000 -5000 0 -0.0001 0 5000 10000 -0.0002 -0.0003 M2=Mx2+My2. M Expt. Expt H Mx=My=M/2 H=0 How a writer works. 2. Ferromagnets 2. Micromagnetics H ~ 2000 Oe H ~ 200 Oe BP Side View gap Coils ~ 20,000 Oe x g •Coils generate about 200 Oe. ABS •Field at the write gap is about 20,000 Oe. •How do we get a 100-fold increase in field strength ? M ~ Cos g Soft Magnetic Material means that M can H easily align with the applied field H. 2. Ferromagnets 2. Micromagnetics Field from coils rotates the magnetic moments Top-down view •Each magnetic grain I=0 I >> 0 acts like a small bar magnet (millions of them). Top •The stray field from Pole each bar H ~ 2Bs acts on neighboring i h l li grains to help align them further. H=0 H~2000 ABS Pole tip t i l C F P l ti material: CoFe H ~20,000 2.4T = 24,000 Oe 1. Overview Bits of Information: Pseudo-Random Sequence. •Information recorded on the medium in a disc drive appears as a sequence random sequence. •Meaningful detection of this data (SNR) is limited by bits that are close together: The high bpi portion of the sequence limits data recovery. High Resolution M ti F Mi Hi h R l ti Magnetic Force Microscopy (MFM)

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